The present invention relates to a simple and inexpensive power supply, and more particularly to a power supply of the type which automatically varies the firing angle of a thyristor in order to control and output voltage. A further aspect of the invention is the provision of a "watchdog" circuit which operates a circuit interrupter if a train of output pulses is terminated or disrupted.
Circuit protection devices such as circuit breakers, relays, contactors and the like are commonly used for disconnecting electrical circuits upon the detection of undesired currents. In addition to breaking the circuits in which the currents flow, other functions may be provided such as actuating alarms and safety devices, or the control of other apparatus in response to a sensed current characteristic. While in principle the opening of an electrical circuit in response to undesirably high currents is a simple procedure, in practice the operation of such protective devices is highly complex owing to the various, often conflicting requirements of electrical systems.
For instance, while it is necessary to protect electric motors from high currents which could damage or destroy the windings, in order to start a motor under load a high initial current is required. Also, during the operation of various electrical equipment, for instance under changing loads, high current flow must be tolerated for short periods of time. Further a single "threshold" for current flow cannot be assigned since a small overcurrent condition can be tolerated far longer than a high overcurrent condition. For these reasons industrial relays and contactors are commonly provided with complex control mechanisms which make use of two or more different current-responsive stages in an attempt to "tailor" the tripping characteristics of the device to a desired application.
In principle it is known that more sophisticated control systems can be designed to replace the present electromechanical, magnetic, and thermal controls. Most such controls must derive their operating voltage from power lines which carry relatively high voltages. For commercial reasons the devices must be usable with a broad range of voltages, often from 100 to 500 volts. The voltage of the control power supply, however, must remain within a closely-regulated band of the order of 10 volts. At the same time the power supply must be simple, rugged and relatively inexpensive.
Recently efforts have been made to design electronic control systems which make use of digital and other allied signal processing techniques which will provide the desired functional flexibility, and eliminate the need for mechanical adaptations to change the range or operating characteristics of a control. Two examples of such a system are shown in U.S. Pat No. 4,219,858--DePuy et al and U.S. Pat. No. 4,219,860--DePuy. These patents disclose an overcurrent relay control which utilizes digital sampling, multiplexing and signal accumulation techniques for detecting overcurrent conditions in one or more phases of a multiphase electrical system.
The nature of the direction exercised over the output device, such as a relay winding, operated by the control system is also a critical factor and, particularly with sophisticated and complex control systems, it is desirable to provide a form of "fail-safe" output control in order to be certain that outputted signals cannot be misread and that improper functioning or anamolous signals in the control system will not result in catastrophic failure. It will therefore be appreciated that it would be highly desirable to provide an improved circuit interrupting control system which supervises interrupter operation in a "fail-safe" manner, and to provide a voltage supply stage which is relatively inexpensive but is capable of accepting applied voltages whose values vary over a wide range.
It is therefore an object of the present invention to provide an improved control operating apparatus for circuit interrupters of the overload relay and contactor type.
Another object is to provide an interrupter control which monitors the status of the interrupter and responds in a "fail safe" mode to control failure.
Another object is to provide an inexpensive solid-state power supply which utilizes elements to convert a broad range of available voltages into a regulated, low-voltage supply.
It is a further object of the invention to provide a low cost phase-angle controlled power supply which automatically converts a wide range of available line voltages into a single, low-level voltage suitable for use with digital control equipment.